Americans consume over 15 billion gallons of soda per year; the equivalent of >550 cans per individual. Caffeine use in adolescents has increased by 70% in the past 30 years. Within this age group, the most popular route of caffeine administration is soda. In addition to potential health risks from consuming caffeine, soda tends to contain high levels of sugar, which puts children at risk for dental caries and overweight. Although it is unclear if caffeine itself is addictive, a subset of adult and adolescent caffeine users meet the Diagnostic and Statistical Manual for Mental Disorders-IV criteria for drug dependence. In addition, caffeine usage increases sensitivity and potentiates addiction to cocaine, amphetamine, and nicotine. Because sugar is a known "natural reward" that activates similar neuronal pathways as drugs of abuse, we hypothesize that the long-term pairing of caffeine with sugar leads to enhanced sugar consumption, potentially mediated by increased reinforcing value of foods and beverages containing added sugar. The studies outlined here will test this hypothesis as well as determine whether adolescents develop physiological and/or psychological tolerance to the effects of caffeine. Children ages 12-17 years will be subdivided into quartiles based on their self-reported daily caffeine intake. This will allow us to assess whether there are different levels of habitual caffeine use lead to changes in physiological and behavioral responses to caffeine administration. In Specific Aim 1, a dose response study will be conducted by taking physiological (heart rate, blood pressure, and hand tremor) and psychological measurements in response to administration of placebo and three doses of caffeine. In addition, the effects of caffeine on consumption of foods and beverages containing added sugar and sensitivity to and liking of sucrose will also be assessed. Specific Aim 2 will measure how chronic and acute caffeine modulates taste preferences for novel drinks. Finally, Specific Aim 3 will assess the relative reinforcing value of caffeinated and non-caffeinated beverages. These studies investigate a novel set of questions concerning the physiological and behavioral consequences of caffeine ingestion among adolescents. In addition, these results will provide information on the potential link between caffeine use and the development of food preferences, in particular preference for foods and beverage with high levels of added sugar. Obtaining NIH funding and completing this set of studies will contribute to the Principal Investigator's career goals by providing training in the areas of human ingestive behavior, behavioral pharmacology, and addiction. These studies are a first step toward establishing an independent line of research that involves studying neuronal mechanisms that regulate ingestive behavior. In addition, this award will demonstrate the funding potential necessary to become an independent investigator and to compete for an RO1 grant.